Force fitting is to be distinguished from shrink fitting which is used in connection with an interference-fitting method for joining entirely different materials having entirely different characteristics. In the latter method a non-resilient part, i.e. a metal, of a two-part assembly is cooled to a low temperature to cause it to shrink, the shrunken male part is then mounted, for example, into a female second part and allowed to expand at room temperature to create the "shrink fit"; see U.S. Pat. Nos. 1,955,728, 1,980,156, 2,038,592, 3,621,550, 4,305,203 and 4,314,396. Shrink fitting is also utilized in the method and apparatus disclosed and claimed in U.S. Pat. No. 3,724,059. The method described in the latter patent separates interference-fitted members by utilizing the contraction properties of metals by subjecting the interior part of the two-part assembly to liquid nitrogen or other suitable refrigerant which shrinks this part while the outer member is expanded by heating. This enables one to easily separate the two parts.
A similar method to shrink fitting is one which utilizes the martensitic transformation characteristics of certain alloys. In this method, pipe couplings of such an alloy are expanded by passing a mandrel through them while they are immersed in a liquid nitrogen (LIN) bath and cooled to a temperature at which a martensitic transformation takes place. When the couplings are warmed to room temperature they return to the austenitic phase, causing them to spring-back to their original shape. This method is used in joining ends of pipes.
In contrast to prior art methods which use either martensitic transformation or thermal expansion/contraction (shrink) fitting, the present invention is an improvement over prior art methods which use a force fitting step in a method for joining, for example, elastomeric materials having orifices therethrough with a second member. Examples of products which have been made by such prior art processes include electrical insulators in which the female member is physically expanded over a solid Teflon.RTM. fluorocarbon mandrel and totally immersed in a dry ice/methanol bath for a five to ten minute period in order to completely cool the elastomeric workpiece to its glass transition temperature, i.e., that region in which the elastomer, either raw or cured, passes from its rubbery, elastic state to a glassy region below which the material no longer displays its elastomeric properties. Such a transition can be determined from the materials thermomechanical curve by determining the temperature dependence of deformation caused by the action of a constant stress under a given temperature-time condition and then plotting the deformation or strain versus the temperature. After the entire female part has reached its glass-transition temperature, it is immediately removed from the bath and mounted onto the male part which can be a metal in the case of friction-fit combinations or an elastomeric core in the case of true interference-fit combinations, e.g. electrical insulators. The prior art method is also useful for mounting gaskets on various rods and the like and in the manufacture of electrical conducting parts which must be insulated or similarly coated with a suitable elastomeric material.
The prior art methods suffer from many inherent inefficiencies including the fact that the entire female member must be cooled to its glass-transition temperature before it can be inserted over the male member; the dry ice/alcohol bath cannot be further used; the carbon dioxide and/or alcohol vapors, if they are allowed to accumulate in the work environment, will create a safety hazard; and the equipment used for such a method tends to be cumbersome.